Variations in normal color vision. IV. Binary hues and hue scaling.

We used hue cancellation and focal naming to compare individual differences in stimuli selected for unique hues (e.g., pure blue or green) and binary hues (e.g., blue-green). Standard models assume that binary hues depend on the component responses of red-green and blue-yellow processes. However, variance was comparable for unique and binary hues, and settings across categories showed little correlation. Thus, the choices for the binary mixtures are poorly predicted by the unique hue settings. Hue scaling was used to compare individual differences both within and between categories. Ratings for distant stimuli were again independent, while neighboring stimuli covaried and revealed clusters near the poles of the LvsM and SvsLM cardinal axes. While individual differences were large, mean focal choices for red, blue-green, yellow-green, and (to a lesser extent) purple fall near the cardinal axes, such that the cardinal axes roughly delineate the boundaries for blue vs. green and yellow vs. green categories. This suggests a weak tie between the cone-opponent axes and the structure of color appearance.